Extension Equine Team

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Bot Flys Are Out!

Posted: July 27, 2014

Three species of bots are considered serious pests to horses. The bot is the larval stage of the horse bot fly, Gastrophilus species. Bot fly species vary in where they lay their eggs on the horse and how the eggs hatch. In general, they lay eggs on the forelegs, shoulders and lips during the summer and fall. Where the eggs are laid describes the three species: common bot fly, chin bot fly and nose bot fly.

Stages of the Bot Fly

The eggs hatch when the horse licks and rubs them. Once in the mouth, the
small bots burrow into the gums and tongue where they remain for about one month
and then migrate into the stomach.

Upon entering the stomach, the larvae attach
themselves to the mucus membrane and remain there for the next eight to 10
months. They detach themselves in the spring and pass through the feces. Once
outside the host, the larvae pupate.

Mature flies emerge in four to six weeks.
The flies mate, the adult females lay eggs, and the cycle begins over again. The
flies do not bite, but egg-laying is annoying to the horse.

The principal damage
caused by the bot is to the stomach lining. In extreme cases, bots can cause a
stomach rupture or fatal colic if they block the stomach valve to the small
intestine.

Management

Consult your veterinarian for drugs available for internal parasite control.
Ivermectin is the de-worming drug of choice; be sure to follow the directions on
the label before using it.

Remove (by clipping or scraping) the yellowish bot
eggs from the horse’s hair before they hatch. Then sweep them up and dispose of
the clippings and eggs away from feeding areas.

Warm water will cause eggs to
hatch, allowing small larvae to be killed before they enter the mouth.